We have isolated a series of novel cyclic peptides from the marine tunicate Lissoclinum patella. A partially purified mixture of peptides arrested the growth of P-388 lymphocytic leukemia in mice (PS test) and several peptides have subsequently been identified as antitumor agents in a variety of tissue culture assays. We have also tested 15 related tunicates and 6 of these exhibited antitumor activity. Because a significant percentage of the tunicates we tested exhibited antitumor activity, we plan a systematic investigation of this group of marine organisms for potential cancer chemo-therapeutic agents. This work has already yielded a new class of antitumor metabolites. Further work will yield additional antitumor metabolites.